Processes for carrying out liquid phase fluorination, in the presence of a catalyst, based on the reaction of hydrofluoric acid and a chlorine-containing organic starting material have been known for many years. They are generally implemented in a plant comprising a reactor, a separation column and a condensor. The chlorine-containing organic starting material and the hydrofluoric acid are supplied continuously to the reactor and the products resulting from the reaction, in other words the fluorinated or chloro-fluorinated organic products and the hydrochloric acid formed are recovered from the plant downstream of the condensor.
By way of examples of chloro-fluorinated organic products to which the invention particularly relates, the following can be mentioned without this list constituting in any way a limitation of the invention:
CFCl.sub.3 . . . fluorotrichloromethane (F11) PA1 CF.sub.2 Cl.sub.2 . . . difluorodichloromethane (F12) PA1 CHClF.sub.2 . . . chlorodifluoromethane (F22) PA1 CFCl.sub.2 --CF.sub.2 Cl . . . 1,1,2-trichloro-1,2,2-trifluoroethane (F113) PA1 CF.sub.2 Cl--CH.sub.2 Cl . . . 1,2-dichloro-1,1-difluoroethane (F132b) PA1 CF.sub.3 --CH.sub.2 Cl . . . 1-chloro-2,2,2-trifluoroethane (F133a) PA1 CF.sub.2 Cl--CH.sub.3 . . . 1-chloro-1,1-difluoroethane (F142b) PA1 CFCl.sub.2 --CH.sub.3 . . . 1,1-dichloro-1-fluoroethane (F141b) PA1 a fluorination reactor supplied with hydrofluoric acid, an organic charge, and a catalyst system; PA1 a line from the head of said reactor and feeding a separation column; PA1 a condensor linked to the head of said column and supplying a gaseous fraction which is recovered, and a liquid fraction which is returned to the head of said column in order to provide reflux thereof; PA1 a collection line located at the bottom of said separation column collecting the heavy fraction at the bottom of said separation column for recycling thereof to the bottom portion of said reactor; PA1 and a recovery line located on said collection line or at an intermediate level comprised between that of a theoretical plate above the level of the bottom of said column and that of a theoretical plate below the feed level of said separation column.
Obviously, the starting materials, the molar ratios of the reagents, the catalyst, the amount of catalyst as well as the temperature and pressure employed for the reaction, and other things as well, will be chosen as a function of the fluorination products that it is desired to obtain. The catalysts employed are, in this type of reaction, generally those comprising halides of groups IVa, IVb, Va, Vb, VIa, VIb and VIII of the Periodic Table.
Particular use for this purpose is made of pentavalent antimony halides, more particularly antimony pentachloride. Their activity is maintained by permanently adding halogen, particularly chlorine. The halogen and the catalyst form the catalytic system.
It should nevertheless be noted that these fluorination reactions are generally accompanied by parasitic reactions such as dehydrochloration, dimerization trimerization etc. followed by fluorination and/or chlorination. Moreover, by-products generated by impurities which may be present in the organic starting materials can also be formed. These by-products, whether they are the result of parasitic reactions or reactions with impurities that are present, are in certain cases heavier, from-the point of view of their vapor pressure, than the fluorination products that it is desired to obtain.
By virtue of the design of the apparatus, and without any particular steps being taken, a part at least of the by-products formed accumulates in the reactor and leads either to a loss of selectivity by acting on liquid-vapor equilibria, or to deactivation of the catalyst.
Faced with this production of by-products and the need to have full control of the composition of the reaction medium, two techniques have been employed to date:
The first of these consists in employing somewhat drastic fluorination conditions so as to eliminate as much as is possible these by-products by fluorinating them and then "purging" or "sweeping" them by the flow of the principal products at the head of the separation column. This is notably the case in the well known processes for producing the F11, F12, F22 and F113 derivatives which are generally carried out in a concentrated catalytic medium.
The second technique consists in eliminating these by-products by means of purging carried out on the reactor. This is notably the case in processes that are carried out in a diluted catalytic medium of which two examples can be cited: production of F142b (cf. French patent 2 652 573) and the production of F133a described by way of example in this present application (comparative example No. 1).
For various reasons, notably selectivity, the first technique consisting in exercising as much control as possible on the composition of the reaction medium by employing conditions highly oriented towards fluorination, cannot always be applied.
Moreover, the second technique involving the checking of the composition of the reaction medium by removal of reactor content suffers from the disadvantage of creating a heavy flow of catalyst that requires treatment in order to eliminate or recycle it.